Ladder Logic Expert Witness

Relay Ladder Logic Expert

There were automated manufacturing machines long before there were digital computers, or even transistors. These machines had their "programming" embedded in hardware. One of the techniques for building control logic into a machine using hardware is called relay logic. A relay is just an electrical switch that is opened and closed using magnetic energy rather than mechanical energy. A relay has two states: closed and open. When the relay is closed electrical  current can flow and when it is open current cannot flow. Thus a relay can be thought of as a single digital element that is either on or off (1 or 0). Machine designers in the time before computers could wire relays together to create digital logic.

Relay ladder logic is a software incarnation of the relay logic that came before it. Machine programmers create graphical software diagrams that represent these logical statements. Ladder logic is called ladder logic because the diagrams resemble ladders with vertical rails on the left and right, and horizontal rungs running sequentially between the rails. Hypothetical electrical current flows through the rungs from the left rail to the right rail.

In keeping with the relay analogy, ladder logic has two main elements: the contact and the coil. The contact is typically represented by two vertical, straight lines and the coil is represented by two vertical lines that curve inward towards each other. When a contact is closed, the hypothetical current can flow through it and energize the coil. In the ladder logic diagram below; X, Y and Z are contacts and A, B, C and D are coils.

We can now analyze the logical statements in the relay ladder logic diagram below. There are two elements in the first rung: X and Y. If the contact X is closed, then the hypothetical current can pass through it and energize the coil A. The logic statement is, "if X, then A." The second rung has three elements: X, Y and B. If the contacts X and Y are both closed, then the hypothetical current can pass through them and energize the contact B. If either X or Y are open, then the current cannot flow to energize B. The logical statement for this is, "if X and Y, then B." The third rung also has three elements, but they are arranged differently. In this arrangement, if either X or Y are closed, then the hypothetical current can flow through to energize C. The logical statement for this rung is "if X or Y, then C." The fourth rung creates the logical statement, "if (X or Y) and Z, then D."

Ladder Logic Diagram


I have professional experience writing relay ladder logic software and expert witness experience analyzing ladder logic software for Rockwell, Omron and Wago PLCs. I can support your litigation efforts as a relay ladder logic expert witness. My qualifications include numerous peer-reviewed publications and over thirty years of engineering experience with software, robotics, instrumentation, medical devices, computer-controlled machines and factory automation. 

I accept a small number of litigation support engagements to complement my regular work as a professional engineer designing automation and machine control systems.


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